Dense nanostructured zirconia by two stage conventional/hybrid microwave sintering

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Abstract

The processing of 3 mol% yttria partially stabilised zirconia nanopowder into components has been investigated via slip casting low viscosity but high solids content nanosuspensions and subsequent pressureless sintering via one and two stage sintering involving both pure conventional heating and hybrid conventional-microwave heating. Very homogeneous and uniform green bodies with densities up to ∼54% of theoretical could be produced, the major limitation being cracking on drying when the highest solid content suspensions were used. This could be partially overcome via the use of humidity drying. The pressureless sintering of the bodies revealed that the two stage sintering process allows a much finer average grain size to be retained than conventional single stage firing, whilst the use of hybrid heating gave further improvements. Greater than 99% dense ceramics with average grain sizes of ∼65 nm could be produced from a powder with an average particle size of ∼16 nm.

Introduction

Nanostructured materials have received much attention in recent years; their appeal is their potential to display unusual physical and mechanical properties such as superplasticity in ceramics at elevated temperatures, transparency for usually opaque materials, controlled band gaps in electronic materials, very high magnetoresistance and superparamagnetic properties, and higher hardness and strength in both metals and ceramics.1 Other benefits include a reduction in the sintering temperatures required, allowing metals and ceramics to be co-fired to a greater extent as well as saving energy, whilst the use of nano-sized components will allow devices to be shrunk significantly in size whilst simultaneously increasing their functionality. However, whilst commercial nanopowders offering these properties have now been produced successfully, sometimes in relatively large quantities, a number of challenges still need to be surmounted if engineering parts are to be manufactured. Whilst ‘bottom up’ approaches may be the long-term solution, these will not be commercially available for several years—and will require industry to completely retool. Therefore there is considerable mileage to be gained by examining what can be achieved practically now using a ‘top down’ approach based on existing manufacturing facilities. If components can be produced without loosing the nanostructure, there is the potential to use the materials for mechanical, thermal, magnetic, electric or electronic applications such as tools, wear and structural parts, magnets, capacitors, varistors and electronic substrates.2

Section snippets

Experimental

The work presented is based on the processing of 3 mol% yttria partially stabilised zirconia (3YSZ) nanpowder with an average particle size of ∼16 nm (Fig. 1). It is produced by MEL Chemicals in the UK as aqueous suspensions of ∼5 vol% solids content.

The as-received suspension was concentrated to yield nanosuspensions with solids contents up to ∼34 vol% but with viscosities as low as ∼0.2 Pa s; details of the process are being published elsewhere and a patent application has been submitted and

Results and discussions

The rheology of the optimised precursor nanosuspension is shown in Fig. 3. It can be seen how the viscosity of the as-received suspension increased very rapidly with concentration, exceeding 1 Pa s by ∼15 vol%. Whilst the use of Dispex A40, together with appropriate exposure to ultrasound to destroy any agglomerates present, allowed much higher solids contents to be achieved whilst retaining a low viscosity, it may be seen the short molecule TAC was even more successful. Up to ∼34 vol% suspensions

Conclusions

It has been possible to concentrate commercial, low solids content aqueous nanosuspensions consisting of ∼16 nm 3YSZ powder particles up to ∼32.5 vol% solids content whilst retaining a viscosity as low as 0.2 Pa s using a new process; a patent application has been submitted. These nanosuspensions have subsequently been slip cast using standard plaster of Paris moulds to yield green bodies that are very uniform and homogeneous. Significant problems with cracking on drying for the samples made from

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    Citation Excerpt :

    To retain tetragonal and cubic phases at room temperature, lower valence oxides (also known as stabilizing agents or dopants, such as CaO, MgO, CeO2 or Y2O3) have been explored [4,36,42–44]These tetragonal and cubic phases are characterized by the replacement of some of the Zr4+ ions in the crystal lattice of zirconia by the slightly larger ions of the dopants presenting also a fraction of oxygen sites vacant to retain charge neutrality stabilization [40]. Due to its excellent performance, yttria-stabilized zirconia (YSZ) is the most extensively used in the literature reports [4,23,41,42,45–164]. Therefore, the features of YSZ strongly depend on structural and phase changes with respect to the Y3+ addition amounts, being corroborated by Y2O3–ZrO2 phase diagrams that tetragonal zirconia is stabilized for low contents of Y2O3, whereas excess Y2O3 additions yield tetragonal and/or cubic ZrO2 mixtures at elevated temperatures [36,41].

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1

Present address: CSIC, Madrid, Spain.

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